Circuitized substrate assembly and method of making same

ABSTRACT

A circuitized substrate assembly and method for making same wherein the assembly includes individual circuitized substrates bonded together. The substrates each include at least one opening, only one of which is substantially filled with a conductive paste prior to bonding. Once bonded, the paste is also partially located within the other opening to provide an effective electrical connection therewith.

TECHNICAL FIELD

[0001] The present invention relates to circuitized substrates, and moreparticularly to composite circuitized structures such as printed circuitboards (PCBs). The present invention also relates to a method forfabricating such structures.

BACKGROUND OF THE INVENTION

[0002] One method of forming a laminate circuitized board structureincludes forming layers of dielectric material and electricallyconducting material to provide multiple layers of circuits and voltageplanes. Circuits can be discrete wiring patterns known as signal planes.Voltage planes can be either ground plane or power planes, and aresometimes collectively referred to as power planes. In one technique offorming such structures, layers of dielectric material and conductivematerial are successively applied, i.e., the dielectric material isapplied and then circuits or voltage planes are provided thereon and, ifnecessary, through holes formed by drilling or etching. This methodrelies on each successive step of adding additional structure and thecircuitry layers are formed individually, e.g., in each step in formingthe plane having circuit traces or formed power planes. This requiresprecision drilling to form the plated through holes (PTHs) all of whichis time consuming, especially where there is a large number of drilledholes required to form PTHs.

[0003] More recently, methods have been described that provide arelatively inexpensive photolithographic technique of forming acomposite laminate structure (substrate assembly) from individualdiscrete laminate structures (substrates). For example, see U.S.application Ser. No. 09/812,261, entitled “Printed Wiring BoardStructure With Z-Axis Interconnections” and filed Mar. 19, 2001. Seealso U.S. Pat. Nos. 6,388,204 (Lauffer et al) and 6,479,093 (Lauffer etal), the teachings of which are incorporated herein by reference.Although the structures and methods of these inventions providesignificant advances and advantages over current PCB fabricationmethods, there still exists a need for further refinement. Therefore,continuing efforts are underway in attempting to provide for evengreater advantages. The present invention is a result of such efforts.

[0004] It is believed that such an invention will constitute asignificant advancement in the art.

OBJECTS AND SUMMARY OF THE INVENTION

[0005] It is a primary object of the present invention to enhance thecircuitized substrate assembly art.

[0006] It is another object of the invention to provide a circuitizedsubstrate assembly which provides for high density wiring patterns andwhich can be manufactured in an efficient, facile manner.

[0007] It is still another object of the invention to provide a methodof making the aforementioned circuitized substrate assembly.

[0008] According to one aspect of the invention, there is provided acircuitized substrate assembly comprising a first circuitized substrateincluding an opening therein, a second circuitized substrate bonded tothe first circuitized substrate and including an opening thereinsubstantially aligned with the opening in the first circuitizedsubstrate, and a quantity of electrically conductive paste substantiallycompletely filling the opening in the second, bonded circuitizedsubstrate and only partly filling the opening in the first circuitizedsubstrate.

[0009] According to another aspect of the invention, there is provided acircuitized substrate assembly comprising a first circuitized substrateincluding an opening therein, a second circuitized substrate bonded tothe first circuitized substrate and including an opening therein alignedwith the opening in the first circuitized substrate, an electricallyconductive cover member substantially covering the opening in the secondcircuitized substrate on the surface of the second circuitized substratefacing the first circuitized substrate, and a quantity of electricallyconductive paste positioned on the cover member and only partly fillingthe opening in the first circuitized substrate.

[0010] According to yet another aspect of the invention, there isprovided a method of making a circuitized substrate assembly comprisingproviding a first circuitized substrate including an opening therein,providing a second circuitized substrate including an opening therein,substantially filling the opening in the second circuitized substratewith a quantity of electrically conductive paste, aligning the first andsecond circuitized substrates such that the opening in the secondcircuitized substrate having the electrically conductive paste thereinaligns with the opening in the first circuitized substrate, and bondingthe first and second circuitized substrates together such that theelectrically conductive paste only partly fills the opening in the firstcircuitized substrate while also substantially filling the opening inthe second circuitized substrate.

[0011] According to still another aspect of the invention, there isprovided a method of making a circuitized substrate assembly comprisingproviding a first circuitized substrate having an opening therein,providing a second circuitized substrate having an opening therein,positioning an electrically conductive cover member on a surface of thesecond circuitized substrate facing the first circuitized substrate tosubstantially cover the opening in the second circuitized substrate,positioning a quantity of electrically conductive paste on the covermember, aligning the first and second circuitized substrates such thatthe openings in the circuitized substrates are substantially aligned,and bonding the first and second circuitized substrates such that thequantity of electrically conductive paste only partly fills the openingin the first circuitized substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a side-elevational view of a circuitized substrateassembly prior to bonding thereof, according to one embodiment of theinvention;

[0013]FIG. 2 is a side-elevational view of the assembly of FIG. 1following bonding;

[0014]FIG. 3 is a side-elevational view of a circuitized substrateassembly prior to bonding thereof, according to another embodiment ofthe invention; and

[0015]FIG. 4 is a side-elevational view of the assembly of FIG. 3following bonding.

BEST MODE FOR CARRYING OUT THE INVENTION

[0016] For a better understanding of the present invention, togetherwith other and further objects, advantages and capabilities thereof,reference is made to the following disclosure and appended claims inconnection with the above-described drawings. It is understood that likenumerals will be used to indicate like elements from FIG. to FIG.

[0017] The following terms will be used herein and are understood tohave the meanings associated therewith. By the term “circuitizedsubstrate” is meant to include substrates including at least onedielectric layer and one conductive layer therein or thereon. Examplesinclude printed circuit boards or like structures made of dielectricmaterials such as fiberglass-reinforced epoxy resins,polytetrafluoroethylene (Teflon), polyimides, polyamides, cyanateresins, photoimageable materials, ceramic and other like materialswherein the conductive layer is a metal layer (e.g., power or signalplane) comprised typically of copper but may include or compriseadditional metals (e.g., nickel, aluminum, etc.) or alloys thereof. Ifthe dielectric is a photoimageable material, it is photoimaged orphotopatterned, and developed to reveal the desired circuit pattern,including the desired opening(s) as defined herein, if required. Thedielectric material may be curtain coated or screen applied, or it maybe supplied as the dry film. Final cure of the photoimageable materialprovides a toughened base of dielectric on which the desired electricalcircuitry is formed. An example of a specific photoimageable dielectriccomposition includes a solids content of from about 86.5 to about 89%,such solids comprising: about 27.44% PKHC, a phenoxy resin; 41.16% ofEpirez 5183, a tetrabromobisphenol A; 22.88% of Epirez SU-8, anoctafunctional epoxy bisphenol A formaldehyde novolac resin; 4.85% UVE1014 photoinitiator; 0.07% ethylviolet dye; 0.03% FC 430, a fluorinatedpolyether nonionic surfactant from 3M Company; 3.85% Aerosil 380, anamorphous silicon dioxide from Degussa to provide the solid content. Asolvent is present from about 11 to about 13.5% of the totalphotoimageable dielectric composition. The dielectric layers taughtherein may be typically about 2 mils to about 4 mils thick. By the term“circuitized substrate assembly” as used herein is meant to include atleast two of such circuitized substrates in a bonded configuration, oneexample of bonding being conventional lamination procedures known in theart. By the term “electrically conductive paste” as used herein is meantto include a bondable (e.g., capable of lamination) conductive materialcapable of being dispensed within openings of the type taught herein.Typical examples of bondable electrically conductive material areconductive pastes such as silver filled epoxy paste obtained from E.I.duPont deNemours under the trade designation CB-100, Ablebond 8175 fromthe Ablestick Company and filled polymeric systems, thermoset orthermoplastic type, containing transient liquid conductive particles orother metal particles such as gold, tin, palladium, copper, alloys, andcombinations thereof. One particular example is coated copper paste.Metal coated polymeric particles disposed in a polymeric matrix can alsobe used. By the term “sticker sheet” as used herein is meant to includedielectric materials such as conventional pre-preg materials used inconventional, multilayered PCB construction, e.g., usually bylamination. Other examples include the products Pyrolux and liquidcrystal polymer (LCP) or other freestanding films. These dielectricsticker sheets may be adhesively applied to one or both of the twocircuitized substrates to assist in bonding these two components. Thesesheets may also be patterned, e.g., by laser or photoimaging, ifdesired. Significantly, such sheets can also include a conductive plane(including signal, ground and/or power) therein to further increase thecircuit density of the finished, bonded product taught herein. Suchsticker sheets may be typically 5 to 8 mils (thousandths) thick.

[0018] In FIG. 1 there is shown a circuitized substrate assembly 10according to one aspect of the invention. Assembly 10 includes first andsecond circuitized substrates 11 and 13, and may further includeadditional circuitized substrates such as substrate 15. In one example,a total of from two to twenty circuitized substrates may be utilized toform a final circuitized substrate assembly in accordance with theteachings of this invention. First substrate 11 includes a plurality oflayers of dielectric material 17 as described above with alternatinglayers of conductive material 19 therein. As stated, such conductivelayers may include signal, ground or power planes of the type typicallyfound in many PCB constructions. Such conductive layers 19 are alsopatterned in accordance with the operational requirements for thefinished assembly. The cross-sectional configurations depicted in thesedrawings are thus representative only and not meant to restrict thescope of the present invention. In one example of the invention, a totalof eight dielectric layers and seven conductive layers may be utilizedto form each substrate 11. The seven conductive layers of this examplemay include four signal layers and three power planes.

[0019] First substrate 11 includes an opening 21 therein. Opening 21extends through the entire thickness of the first substrate 11. In FIG.1, opening 21 further includes a conductive (e.g., copper) plating 23which extends along the internal walls of opening 21 and outwardly ontothe external opposing surfaces of substrate 11. These external portionsof layer 23 may be referred to as lands, pads or the like for eventualelectrical connection to another conductive member (in the case ofsubstrate 13, to other lands). Opening 21 preferably includes a diameterof from about 1 to about 15 mils. The remaining openings describedherein also each preferably include a similar diameter.

[0020] Second substrate 13 is similar to first substrate 11, includinghaving dielectric layers 17 and conductive layers 19 as part thereof.Second substrate 13 also includes an opening 21 similar to that insubstrate 11. Further, substrate 13 includes a conductive layer 23similar also to that of layer 23 in FIG. 1. Layer 23 may also includethe external lands (or pads) of the type shown for substrate 11. In oneexample, both conductive layers 23, and the external lands may possess athickness of from about 0.5 mils to about 3.0 mils.

[0021] Significantly, opening 21 in second substrate 13 further includesa quantity of electrically conductive paste 25 therein. As see in FIG.1, paste 25, of the type defined in detail above, completely fillsopening 21 in the second substrate. Additionally, the electricallyconductive paste 25 may also extend outwardly onto the external surfacesof the lands of layer 23 as also shown in FIG. 1. In one example, atotal of 400 cubic mils of paste may be applied to each external land onboth surfaces of the substrate. Paste application can be performed byconventional techniques such as stencil printing, screen printing orinjection. Further description is thus not believed necessary.

[0022] In the broadest embodiment of the invention, only two circuitizedsubstrates may be utilized to perform a final, bonded assembly 10. Asshown in FIG. 1, however, the utilization of a third circuitizedsubstrate 15 will also be described but is not meant to limit the scopeof the claimed invention. Substrate 15 is thus substantially similar incomponents as substrates 11 and 13. It is also to be noted that althoughthe patterns of dielectric and conductive layers within the varioussubstrates shown are similar, the invention is not limited to theseconfigurations. Several different patterns of layers can be utilized forthe various substrates.

[0023] With the substrates aligned in the orientation shown in FIG. 1,the next step is to bond the substrates to form assembly 10. Such abonded assembly 10 is shown in FIG. 2. The preferred method ofaccomplishing such bonding is to use a conventional laminating process,which, in one example, is carried out at temperatures from about 70 toabout 200 degrees Celsius (C.) for a time period of about 30 to about180 minutes and under a laminating pressure of from about 50 to about500 pounds per square inch (p.s.i.).

[0024] In a preferred embodiment, a sticker sheet 31 is positionedbetween the respective substrates prior to lamination for the purpose ofimproving the bond between the substrates. The preferred sticker sheetmaterial is defined hereinabove. It is understood, according to thebroad aspects of the invention, that use of such a sticker sheet is notessential to achieve a satisfactory bond between the circuitizedsubstrates taught herein.

[0025] In FIG. 2, the bonded circuitized substrate assembly 10 is fullycompressed as a result of the aforedefined lamination process.Significantly, the conductive paste 25 has extended from its originalposition both upwardly and downwardly to partially fill the respectiveopposing openings 21 in substrates 11 and 15. Of further significance,paste 25 also extends between the opposing faces of the respective landsof layers 23 formed on the external surfaces of the respectivesubstrates. Thus, an effective electrical connection has been attainedbetween the conductive layers 23 of all three substrates using but asingular quantity of conductive paste 25. Such interim placement ofconductive paste 25 between the lands further enhances the electricalconnection between the respective substrates in the manner taughtherein.

[0026] Additional operations may now be performed on the bonded assembly10, including the addition of another dielectric layer 33 and aconductive land 35 or the like which extends downwardly to contact theexposed land portion of a respective land portion of layer 23. Layer 33may serve as a mounting layer or the like for additional electricalcomponents (not shown) which may be positioned thereon.

[0027] In FIG. 3 there is shown a bonded circuitized substrate assembly10′ according to an alternative embodiment of the invention. Assembly10′ includes the same components as shown in the embodiment of FIG. 1with the exception that conductive paste is not filled within theopening 21 of substrate 13. Instead, a cover member 41 is provided overthe external land portions of layer 23 of substrate 13. A preferredcover member is a metallic layer, e.g., copper. Prior to positioning ofcover member 41, the opening in substrate 13 is preferably filled with adielectric or other material (including conductive) 43. In a preferredembodiment, material 43 is a dielectric material of the type describedabove. With such dielectric material in place, cover members 41 may beplated over the external exposed surfaces of the positioned dielectric43. In on example, cover layer 41 may include a thickness of from about0.1 mils to about 2.0 mils.

[0028] As further seen in FIG. 3, a quantity of conductive paste 25 ispositioned over the cover member 41 and is comprised of a similar pasteas used in the embodiment of FIG. 1. In one example, a quantity of 400cubic mils of paste 25 is applied onto cover member 41 and will possessa thickness of only about 2 mils. In FIG. 3, two quantities of paste areutilized, but it is again understood that in the broadest aspects ofthis invention, only two substrates (11 and 13) may be utilized to forma successfully bonded final structure 10′.

[0029] In FIG. 4, the exploded structure 10′ in FIG. 3 is shown as beingbonded, preferably using a lamination process as described above.Significantly, the respective quantities of paste 25 have extendedoutwardly from the cover member 41 to partially fill the opening in therespective opposing substrate. Of further significance, a portion of theconductive paste 25 remains positioned between the external surfaces ofthe outwardly projecting land portions of the respective layers 23 toeven further enhance the electrical coupling between the respectivesubstrates. As in the embodiment of FIG. 1, a dielectric 33′, inaddition to an external conductor 35′, may also be added, including onopposite sides of the formed assembly 10′.

[0030] Thus there has been shown and defined a circuitized substrateassembly comprised of at least two circuitized substrates, eachincluding an opening therein which are aligned such that these openingscan be electrically coupled during the subsequent bonding process.

[0031] An assembly of the present invention provides significant wiringdensity increases over conventional printed circuit board constructions.This is primarily due to the fact that wiring PTHs no longer need to gothrough the entire assembly, but rather can stop on any given layer.Thus, wiring on the lower layers can be run unobstructed from anyplated-thru-hole (PTH) blockages. The present invention also eliminatesthe drill and plate concerns of high aspect ratio thru holes that arecommon with conventional circuit boards.

[0032] The present invention provides a robust process and structure,while not requiring complete initial filling of all openings with pasteprior to final joining, which would in turn require more exactingprocess tolerances to achieve high joining yield and reliablestructures. For example, such fillings would require the thickness ofthe 2 conductive pads to be joined plus the thickness of the conductiveadhesive to be roughly equal to the sticker sheet. If the sticker sheetis thicker, then there could be poor contact of the conductive joints.If the pads plus paste are thicker than the sticker sheet, then therecould be voids in the sticker sheet, or the conductive paste could besqueezed laterally, to the extent of possibly shorting adjacent pads.For such requirements, tolerance requirements were less than 1 mil,while the sum of the tolerance capabilities was significantly higherthan 1 mil. The present invention substantially overcomes theseproblems.

[0033] While there have been shown and described what are at present thepreferred embodiments of the invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the scope of the invention as defined bythe appended claims.

What is claimed is:
 1. A circuitized substrate assembly comprising: afirst circuitized substrate including an opening therein; a secondcircuitized substrate bonded to said first circuitized substrate andincluding an opening therein substantially aligned with said opening insaid first circuitized substrate; and a quantity of electricallyconductive paste substantially completely filling said openings in saidsecond, bonded circuitized substrate and only partly filling saidopening in said first circuitized substrate.
 2. The assembly of claim 1wherein said first and second circuitized substrates each include atleast one electrically conductive plane and at least one dielectriclayer.
 3. The assembly of claim 1 wherein each of said openings in saidfirst and second circuitized substrates includes an electricallyconductive material thereon, said electrically conductive paste being inelectrical contact with said electrically conductive material in both ofsaid openings.
 4. The assembly of claim 3 wherein said electricallyconductive material comprises a plated layer of metal.
 5. The assemblyof claim 1 further including an interim dielectric layer between saidfirst and second circuitized substrates.
 6. The assembly of claim 1further including a layer of dielectric material positioned on anexternal surface of said first circuitized substrate and extendingwithin said opening of said first circuitized substrate.
 7. The assemblyof claim 1 further including a third circuitized substrate also havingan opening therein and bonded to said second circuitized substrate suchthat said opening in said third circuitized substrate is substantiallyaligned with said opening in said second circuitized substrate, saidquantity of electrically conductive paste also only partly filling saidopening in said third circuitized substrate.
 8. A circuitized substrateassembly comprising: a first circuitized substrate including an openingtherein; a second circuitized substrate bonded to said first circuitizedsubstrate and including an opening therein aligned with said opening insaid first circuitized substrate; an electrically conductive covermember substantially covering said opening in said second circuitizedsubstrate on the surface of said second circuitized substrate facingsaid first circuitized substrate; and a quantity of electricallyconductive paste positioned on said cover member and only partly fillingsaid opening in said first circuitized substrate.
 9. The assembly ofclaim 8 wherein said first and second circuitized substrates eachinclude at least one electrically conductive plane and at least onedielectric layer.
 10. The assembly of claim 8 wherein each of saidopenings in said first and second circuitized substrates includes anelectrically conductive material thereon, said electrically conductivepaste being in electrical contact with said electrically conductivematerial in said opening in said first circuitized substrate.
 11. Theassembly of claim 10 wherein said electrically conductive materialcomprises a plated layer of metal.
 12. The assembly of claim 8 furtherincluding an interim dielectric layer between said first and secondcircuitized substrates.
 13. The assembly of claim 8 further including alayer of dielectric material positioned on an external surface of saidfirst circuitized substrate and extending within said opening of saidfirst circuitized substrate.
 14. The assembly of claim 8 wherein saidcover member comprises a metallic layer.
 15. The assembly of claim 14wherein said metallic layer is copper.
 16. The assembly of claim 8wherein said opening in said second circuitized substrate is filled witha dielectric material, said cover member being positioned on saiddielectric material.
 17. The assembly of claim 8 further including athird circuitized substrate also having an opening therein and bonded tosaid second circuitized substrate such that said opening in said thirdcircuitized substrate is substantially aligned with said opening in saidsecond circuitized substrate, said assembly further including a secondcover member substantially covering said opening in said secondcircuitized substrate and a second quantity of electrically conductivepaste positioned on said second cover member and only partly fillingsaid opening in said third circuitized substrate.
 18. A method of makinga circuitized substrate assembly, said method comprising: providing afirst circuitized substrate including an opening therein; providing asecond circuitized substrate including an opening therein; substantiallyfilling said opening in said second circuitized substrate with aquantity of electrically conductive paste; aligning said first andsecond circuitized substrates such that said opening in said secondcircuitized substrate having said electrically conductive paste thereinaligns with said opening in said first circuitized substrate; andbonding said first and second circuitized substrates together such thatsaid electrically conductive paste only partly fills said opening insaid first circuitized substrate while also substantially filling saidopening in said second circuitized substrate.
 19. The method of claim 18wherein said bonding of said first and second circuitized substrates isachieved by laminating.
 20. A method of making a circuitized substrateassembly, said method comprising: providing a first circuitizedsubstrate having an opening therein; providing a second circuitizedsubstrate having an opening therein; positioning an electricallyconductive cover member on a surface of said second circuitizedsubstrate facing said first circuitized substrate to substantially coversaid opening in said second circuitized substrate; positioning aquantity of electrically conductive paste on said cover member; aligningsaid first and second circuitized substrates such that said openings insaid circuitized substrates are substantially aligned; and bonding saidfirst and second circuitized substrates such that said quantity ofelectrically conductive paste only partly fills said opening in saidfirst circuitized substrate.
 21. The method of claim 20 wherein saidbonding of said first and second substrates is achieved by laminating.